RNA plays an important role in cellular regulation -- either by its presence in active form or by its total absence. It has been known for several years that transcription of DNA does not necessarily lead to productive, mature RNA molecures. Cleavage of these RNA molecules often is required for the RNA molecules to mature or to act as an intermediate in other processes (e.g., priming of DNA replication). These cleavage events are a subset of a general maturation pathway known as RNA processing. Work of this Intramural Research Project is concerned with two types of RNA processing, generation of RNA primers for DNA replication and ribosomal RNA processing in higher eukaryotes. A mutant of rnh (Ribonuclease H) of E. coli is poorly viable in combination with recBC mutants. We have taken advantage of this observation to construct a double mutant (rnh, recBts) that is temperature sensitive for growth. Screening DNAs from a variety of sources has permitted us to clone (1) mutant E. coli rnh genes (2) a Salmonella ribonuclease H gene and (3) a DNA from Saccharomyces cervisiae. In a second area, we have demonstrated that U3RNA (a nuclear small RNA) can form relatively stable complexes with rRNA precursors. The latter observation supports the idea that U3RNA is involved in rRNA processing.